The present invention is a process for the preparation of acyloxysilanes. The process comprises contacting a chlorosilane with a carboxylic acid in a film forming an equilibrium mixture of acyloxysilane and hydrogen chloride. The film is heated at a temperature sufficient to cause vaporization of the hydrogen chloride from the equilibrium mixture thereby increasing the yield of acyloxysilane in the equilibrium mixture. In a preferred process, the process is run in a falling-film type reactor or a wiped-film type reactor.
Acyloxysilanes are well known cross-linking agents for one-part room temperature vulcanizable silicone rubber compositions. Common acyloxysilane cross-linking agents include methyltriacetoxysilane and ethyltriacetoxysilane.
Acyloxysilane cross-linking agents have been made by the reaction of an appropriate chlorosilane with a carboxylic anhydride or with a carboxylic acid. The process is known to be an equilibrium reaction as described by the equation EQU SiCl+AcOH.revreaction.SiOAc+HCl,
where Ac is an acyl radical.
A reactive column distillation process for preparing acyloxysilanes by reacting a chlorosilane with a carboxylic acid or carboxylic acid anhydride in the presence of an iron complexing agent is disclosed in Ashby, U.S. Pat. No. 3,974,198, issued Aug. 10, 1976. Ashby describes a process where an aliphatic carboxylic acid, such as glacial acetic acid, is added at the top of a distilling column to a refluxing mixture of chlorosilane in an organic solvent, such as benzene, containing an iron complexing agent. After the completion of the addition of the aliphatic carboxylic acid, the solvent is removed by distillation, and the acyloxysilane compound is isolated from the mixture.
John et al., U.S. Pat. No. 4,176,130, issued Nov. 27, 1979, describe a reactive distillation column process where an aliphatic carboxylic acid in the vapor phase is passed upwards from the bottom of a column filled with Raschig rings countercurrent to a flow of chlorosilanes.
Tolentino, U.S. Pat. No. 4,332,956, issued Jun. 1, 1982, describes a reactive distillation column process where acyloxysilanes are prepared by reacting a chlorosilane with an aliphatic carboxylic acid in a column at an elevated temperature where the aliphatic carboxylic acid in the vapor phase reacts with the chlorosilane in the column. The improvement described by Tolentino comprises introducing the carboxylic acid into the column in a plurality of feed streams, where at least one feed stream of aliphatic carboxylic acid is located above the chlorosilane feed stream and one carboxylic acid feed stream below the chlorosilane feed stream.
In such reactive distillation processes as described above, the formation of dimer at the bottom of the column is a significant side reaction that results from thermal decomposition of monomeric acyloxysilanes or from a reaction of the chlorosilane with acyloxysilane or both. When this happens the yield of acyloxysilane product is reduced. Also, gels can form in the reactive distillation column causing plugging of the column. These detrimental processes are catalyzed by the presence of hydrogen chloride in the reaction mixture as well as by excess acetic acid.
The conduct of the reaction as a film process provides for good heat transfer to the film and good mass transfer thereby effecting rapid removal of hydrogen chloride from the reaction mixture. This results not only in a shifting of the equilibrium of the reaction to favor formation of the acyloxysilanes, but also a reduction in the undesired dimer formation catalyzed by the hydrogen chloride. In addition, the conduct of the reaction as a film process allows the use of lower reactor surface temperatures and shorter residence times than those required in reactive distillation columns. A reduction in reactor surface temperatures and residence times can reduce undesired side reactions, such as dimer formation and gelation.